The invention relates generally to ion mobility spectrometry and, more specifically, to ion mobility spectrometers having a window providing a vacuum barrier between a source chamber and a reaction chamber.
In one type of ion mobility spectrometer (IMS), an evacuated electron source chamber contains a non-radioactive electron source. The electron source and an x-ray anode are connected to an accelerating voltage source in such a way that electrons from the source impinge upon the anode, causing the generation of x-ray radiation. These x-rays enter an adjacent reaction chamber of the IMS through a gas-tight x-ray window which is impermeable to electrons from the electron source. The x-rays passing through the window then react with and ionize the molecules of a sample material in the reaction chamber. This type of IMS is known from U.S. patent Ser. No. 09/617,716, which is incorporated herein by reference in its entirety.
In accordance with the present invention, an ion mobility spectrometer is provided that uses a gas-tight window between an evacuated electron source chamber and a reaction chamber, and that provides the window with a support grid attached to the reaction chamber side of the window. Within the electron source chamber is a non-radioactive electron source connected to the negative side of an accelerating voltage source, and an x-ray anode connected to the positive side of the accelerating voltage. The operation of the electron source and the anode are such that x-ray radiation is generated by electrons from the electron source impinging upon the anode. The x-ray radiation passes through the window into the reaction chamber, where it is ionizes material therein. In the invention, the window is stabilized by the support grid, allowing the window to be thinner and of a greater diameter than would otherwise be possible. Preferably, there is a permanent metal bond between the support grid and the x-ray window. Substantially no electrons from the electron source impinge on the support grid, as might otherwise cause undesirable bremsstrahlung. However, in an illustrative embodiment, x-ray radiation does pass through the window to impinge on the support grid and may produce desired photoelectrons for ionization in the reaction chamber.
The x-ray window may comprise beryllium and may have a thickness of between 5 xcexcm and 50 xcexcm and an effective diameter of between 3 mm and 20 mm. Beryllium is traditionally used as window material in x-ray applications because of its low atomic number. At the stated thicknesses or diameters, the mechanical stability of the window without a support grid could fail at a pressure differential as low as approximately 1 bar.
In one embodiment, the anode in the electron source chamber is positioned relative to the x-ray window such that none of the electrons emerging from the electron source reach the x-ray window. This is achieved, for example, by an arrangement where the electrons are accelerated approximately parallel to the partition toward the anode where they arrive at an angle of approximately 45xc2x0 and produce the x-ray radiation (characteristic radiation and/or bremsstrahlung). Only x-ray radiation impacts on the x-ray window, which is therefore unaffected by electrons.
In another embodiment, the x-ray anode may be attached to the vacuum side (i.e., the electron source side) of the x-ray window as a thin layer (e.g., less than 500 nm) so that electrons arriving from the electron source are decelerated in the metal layer and produce x-ray radiation that passes through the x-ray window. In one embodiment, the thickness of such a metal layer is at least seven half-value thicknesses of the electrons penetrating from the electron source, so that substantially no electrons reach the x-ray window directly. In addition, the thermal load is significantly moderated due to the conductivity of the metal layer. It may also be desirable to make the metal layer thin enough that it does not exceed two half-value thicknesses of the x-ray radiation produced. This ensures that the x-ray radiation penetrating through the x-ray window into the reaction chamber is still sufficiently intense. In such an embodiment, the support grid on the other side of the window does not disturb the coating of anode material, or interfere with its application.
The anode material may comprise metals with high atomic number such as tungsten or gold. In such a case, bremsstrahlung is predominantly exploited. However, light elements may also be used, such as aluminum or magnesium, whose characteristic radiation is within a very favorable range so that air components in the reaction chamber, predominantly nitrogen and oxygen, are ionized via their K shells at approx 400 to 500 eV with a large cross-section.
The preferred accelerating voltage is less than 5 kV. This energy level should be sufficient to generate x-ray radiation that penetrates the window and is able to achieve ionization in the reaction chamber, either directly or via photoelectrons. The range in air at atmospheric pressure is largely adapted to the geometric dimensions of the reaction chamber (roughly in the centimeter range). Moreover, these voltage levels can be handled easily and without the need for extreme safety precautions.